1. Field of the Invention
The present invention relates to an electrophotographic apparatus (recording apparatus of an electrophotographic method) which is used in a printer, a facsimile apparatus, a copier, or the like, and more particularly to an electrophotographic apparatus of a potential dividing development method which is a development method wherein an image is developed by using colored particles such as a toner, and in which, as in the case of two-color printing, electrostatic latent images are formed with dividing the surface potential of a photosensitive member into plural levels (so-called potential division) and the electrostatic latent images are developed by toners of different colors that are oppositely charged.
2. Description of the Related Art
A conventional development method and a potential dividing development method which are used in a conventional electrophotographic apparatus will be described.
A recording apparatus using the electrophotographic method implements the steps of: forming an electrostatic latent image on a recording member (photosensitive member); attaching colored particles to the electrostatic latent image in the surface of the recording member to develop the latent image as an image; transferring the developed colored particle image to another recording member; and fixing the transferred colored particle image. As the colored particles, powder dedicated to electrophotography and called a toner is used.
In the developing step, first, toner particles serving as colored particles are charged by using a developer in the following manner. The developer is a mixed powder of a toner and carrier beads which are magnetic particles. The developer is loaded into a developing unit, and then stirred therein to be charged by friction with the carrier beads.
In the developing process, the whole surface of the photosensitive member is once charged, and the photosensitive member is then irradiated (exposed) with light to cause partial discharging in accordance with image data. As a result, a potential contrast caused by a charged region and a discharged region is formed in the surface of the photosensitive member. The potential contrast is called an electrostatic latent image.
The image formation in the developing step is performed by moving the electrostatic latent image on the surface of the photosensitive member to a position (developing position) which is opposed to a magnet roller called a developing roller and transporting the developer. In the image forming process, a method called a bias development is usually used.
In the bias development, a bias voltage is applied to the developing roller to produce an electric field between the latent image potential formed in the surface of the photosensitive member and the developing roller, and the charged toner particles are separated from the developer on the surface of the developing roller to be moved to the surface of the photosensitive member by the function of the electric field, thereby forming an image. The difference between the bias voltage of the developing roller and the potential of the image forming portion of the photosensitive member is called the development potential difference.
It is a matter of course that, when the development potential difference is large, the formed electric field (called the development electric field) is enhanced and hence the developing performance is improved. Also when a method in which the distance between the developing roller and the photosensitive member is reduced, or that in which the electric resistance of the developer is lowered is employed, the same effect of enhancing the development electric field is attained so as to improve the developing performance.
As a modification of the above-described electrophotography bias development method, a development method has been proposed in, for example, JP-A-48-37148. In the proposed method, the potential of a charged region and a discharged region is divided by two, an intermediate potential region is disposed between the regions, a first developing unit which develops the discharged region (this development is called reversal development) is disposed to perform development using a first toner, and a second developing unit which develops the charged region (this development is called normal development) is disposed to perform development using a second toner, whereby toner developments of two kinds are conducted by one charging step and one light-irradiating step (exposing step).
In this development method, the toner is not attached to the intermediate potential region (referred to as the intermediate potential) on the photosensitive member and having a voltage which is between the bias voltage of the reversal developing unit (first developing unit) and that of the normal developing unit (second developing unit), to form the background portion as an image. Therefore, it is possible to form an image of two kinds of toners and composed of the background portion, a first image portion, and a second image portion. In the specification, this development method is referred to as the potential dividing development method.
In the potential dividing development method, generally, the two kinds of toners are configured by toners of different colors, respectively, and used for obtaining an image composed of two colors. Alternatively, the first development may be performed as the normal development, and the second as the reversal development. Also in the alternative, the potential dividing development is enabled.
In the bias development, when an electrostatic latent image is formed on a photosensitive member, an electric field emphasizing the development in end portions of the potentials (potential edges) of the latent image is produced, and at the same time an electric field of a polarity opposite to that of the latent image (reverse electric field) is produced in the periphery of the latent image. This reverse electric field is produced because the development is performed by an electric field which is formed by spatially differentiating the potential distribution, and hence a field emphasized region and an electric field the polarity of which is opposite to that of the emphasized region are produced in potential edges (the boundary between the potential of the charged or discharged region serving as the latent image region and the intermediate potential) of the image regions where the development potential is changed.
In the bias development in which the potential dividing development method is not used and only one kind of toner is used, the reverse electric field does not present a problem. By contrast, in the electric potential dividing development process in which positive and negative electrostatic latent images are formed on the photosensitive member, and development is performed by using two kinds of toners (hereinafter, description will be made with taking red toner and black toner as an example) that are charged in polarities respectively opposite to the polarities of the latent images, fringe development occurs in which the red toner is attached to the periphery of a black image and the black toner is attached to that of a red image by attraction due to the reverse electric fields.
The fringe development appears as erroneous printing in which toner is attached to an unexpected position. In order to solve this problem, a method may be employed in which an electric potential dividing development method without fringe development is realized by introducing an exposure control (hereinafter, often referred to as fringe control) wherein an expected position of occurrence of fringe development is predicted and auxiliary exposure is applied to the expected position to suppress a reverse electric field. In the auxiliary exposure, exposure is performed while controlling the auxiliary exposure amount so as to stepwise relax the development potential difference (the difference between the potential of the charged or discharged region and the intermediate potential) adjacent to the development potential edge of a latent image (image region). This will be described in detail with reference to the drawings in the paragraph of embodiments of the invention.
The auxiliary exposure is applied to both or one of the fringe surrounding the red image and that surrounding the black image. When the fringe control is used, there arises a further problem in that a reverse electric field generated by the potential difference (the difference between the potential of an auxiliary exposure region and the intermediate potential) due to auxiliary exposure causes additional fringe-like erroneous printing.
In the specification, fringe-like erroneous printing caused by auxiliary exposure is called a repulsive fringe. In order to distinguish from a repulsive fringe, a fringe which is caused when auxiliary exposure is not applied is called a primary fringe. In the following description, when referred to merely as fringes, it generally means both the kinds of fringes.
In the fringe control, there is a Wade-off relationship in which, as the difference between the potential due to auxiliary exposure and the intermediate potential is larger (in other words, as the difference between the potential of a latent image region and the potential due to auxiliary exposure is smaller), a primary fringe is more susceptible to extinguish, and a repulsive fringe is correspondingly more susceptible to be produced. When, in order to simultaneously prevent primary and repulsive fringes from occurring, the auxiliary exposure amount is set so that the fringes are substantially equal in degree to each other, the fringe control can attain the maximum effect. Because of this consideration for a repulsive fringe, the effect of the fringe control has its upper limit, and there is a problem in that, even when primary and repulsive fringes are made equal in degree to each other, fringes sometimes occur according to the situation. In the case where only one kind of the auxiliary exposure amount can be applied, the difference between the potential due to auxiliary exposure and the intermediate potential cannot be set to a very small degree (can be set to about one half of the difference between the latent image potential and the intermediate potential), and it may be therefore impossible to sufficiently suppress a repulsive fringe. As a countermeasure, the amount of light of auxiliary exposure may be set to have either of multi levels so that the potential due to auxiliary exposure is made stepwise closer to the intermediate potential as the exposed position is more remote from the position where a primary fringe is suppressed. According to this countermeasure, a repulsive fringe can be suppressed, and, when the auxiliary exposure amount is set in this state so that primary and repulsive fringes are made equal in degree to each other, the effect of the fringe control is further enhanced.
In the above-described fringe control of the potential dividing development method, the optimum measure for specifically realizing the multi-leveled amount of auxiliary exposure is not shown, and a driver for a light emitting element for the exposure must support a plurality of exposure amounts. Therefore, such a control has a problem in that the number of parts, the size, and the cost of a circuit for the driver (hereinafter, such factors are referred to as the circuit scale) are increased. In a case such as that where a semiconductor laser is used as the light emitting element and the process speed is high, particularly, a driver is required for each of the auxiliary exposure amounts, and a problem is produced in that the circuit scale is largely increased.
It is an object of the invention to provide a method of multi-leveling the auxiliary exposure amount which requires only a small circuit scale in the fringe control of the potential dividing development method.
It is another object of the invention to provide an excellent fringe suppressing performance which is free from a ghost, in a fringe controlling method using the multi-leveling methods.
In order to attain the objects, the invention basically provides an electrophotographic apparatus using a potential dividing development method in which developing processes respectively using colored particles of different kinds are performed by normal development and reversal development on two potential levels excluding an intermediate potential level, wherein
the apparatus comprises an auxiliary exposing unit for forming a fringe development preventing potential in an edge of an image forming region in order to prevent a fringe development from occurring, and the auxiliary exposing unit sets an auxiliary exposure amount by performing pulse-width modulation on a laser driver.
In order to achieve an excellent fringe suppressing performance which is free from a ghost, an apparatus is proposed in which the amount of light emitted from a light source is set by using a driver for auxiliary exposure, and a light amount adjusting unit based on the pulse-width modulation method is further employed.
Furthermore, an apparatus is proposed in which at least one of auxiliary exposure lines in a sub scanning direction is different in pulse arrangement from adjacent auxiliary exposure lines.
As an auxiliary exposure unit for setting multi-level exposure amounts (two or more levels), a laser driver dedicated to auxiliary exposure may be used and a driving pulse for the driver may be pulse-width modulated, or a laser driver for forming the intermediate potential or that for forming a reversal image in which the exposure amount can be controlled may be used also for performing the pulse width modulation. In the latter case, for example, the laser driver for forming the intermediate potential may be used also for auxiliary exposure on a normal image, and the laser driver for reversal development may be used also for auxiliary exposure for forming a reversal image. Alternatively, the laser driver for reversal development may be used also for both auxiliary exposure on a normal image and that on a reversal image, and multi-level auxiliary exposure may be ensured by pulse-width modulation.